Speaking the language of substations: standardized communication protocols in Vaisala’s DGA Monitor

Making sure different bits of equipment can talk to each other isn’t always easy, so the Vaisala Optimus™ DGA Monitor OPT100   supports three of the most popular communication protocols – helping the vital data it gathers from electricity substations get to where it needs to be.

When the OPT100 monitor is installed in an electricity substation, it is connected to the substation’s SCADA (Supervisory Control and Data Acquisition) system. This system processes the measurement data gathered by the monitor – typically between 10 and 50 measurement values from each oil analysis. The measurement results are sent to the SCADA using machine-to-machine communication protocols.

In practice the SCADA system requests information from the monitor periodically, for example once per second. As well as measurement results, this information can include things like alarms and system status.

There are several different communication protocols available for SCADA communication. In the past manufacturers often defined their own, which led to compatibility issues between substation network components. Thankfully, standardization has narrowed the number of different protocols in use, thus improving interoperability. If substation components do not support the same protocols as the substation’s SCADA system, a protocol converter will be needed.

The OPT100 supports three of the most popular communication protocols: Modbus, DNP3, and IEC 61850. While they can all be used to transfer basic information from the OPT100 to the SCADA system, they differ slightly in terms of functionality and operation. Let’s take a look at each of the three protocols in a bit more detail.

Modbus

The oldest of the three protocols, Modbus was defined in the 1970s, and despite its modest set of features it remains widely used. It is a general-purpose protocol with no substation-specific features. Modbus can be implemented in several different ways. The OPT100 offers the two most common ones: Modbus RTU with RS-485 as a hardware layer and Modbus TCP over Ethernet. Modbus communication is based on the master-slave principle. The master initiates communication and the slave replies to messages addressed to it. In an electricity substation the SCADA is the master and requests information from the slave OPT100 by reading the Modbus registers.

Setting up Modbus communication requires a lot of manual work. The Modbus interface description (often called a register map) is a human-readable document explaining what Modbus functions are implemented and what information is mapped to which register and in what data format. When a Modbus device is used with a SCADA system, the installation technician must manually configure the system using data from the Modbus register map.

DNP3

The DNP3 protocol was defined in the 1990s and is designed specifically for electricity substations and other utility applications, such as water treatment and gas supply. It is mainly used in the Americas and in some Asian countries such as Japan.
DNP3 has been defined for serial communication and Ethernet interfaces, and has several functions supporting substation-specific features. The DNP3 interface is described in the device profile document delivered with the device, and explains all communication parameters, available functions, and measurement information. The profile document can be viewed in human-readable format or uploaded to the SCADA system for easier configuration.
With the DNP3 protocol, the SCADA system is a master station and the OPT100 an outstation. DNP3 is mostly used in master-slave mode, but DNP3 outstations can also initiate certain kinds of communication such as sending alarms.

OPT100 offers an Ethernet-based DNP3 interface - a detailed description can be found in the OPT100 DNP3 Device Profile document.

IEC 61850

IEC 61850 is a standard for the design of substation automation developed by the International Electrotechnical Commission (IEC), and is often seen as a future-proof solution. It contains reference architecture and several protocols and data models, and is much more complex than both the Modbus and DNP3 protocols. The main idea is that all substation components – transformers, switches, relays, measurement equipment, communication links, configurations, and so on – are described using SCL (Substation Configuration description Language). The goal is to provide a manufacturer-independent way to describe and operate electrical substations.
In an IEC 61850 context, the OPT100 is called an Intelligent Electronic Device (IED). An IED always contains a set of SCL files describing it. In practice, the substation designer imports an SCL file to the substation database and configures the necessary parameters, then the configured file is uploaded to the OPT100 and becomes a logical part of the substation’s functionality. The IEC 61850 user group can be found at www.iec61850.ucaiug.org.

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